(1) Field of the Invention
The present invention generally relates to a microwave oven abnormal state detecting device and a method of detecting an abnormal state of a microwave oven. More particularly, it relates to a microwave oven abnormal state detecting device that is capable of monitoring either an unloaded state of a cooking chamber or an overheating of a magnetron or a cooking chamber by the use of a voltage applied to both terminals of the magnetron, and further relates to a method of detecting an abnormal state of a microwave oven.
(2) Description of the Prior Art
An overheat cutoff switch such as a therm-switch has been generally employed in each of a magnetron and a cooking chamber of a microwave oven to provide protection to microwave oven components against overheating of the magnetron and cooking chamber.
There is U.S. Pat. No. 5,575,943 as a prior art relating to a safety mechanism for a microwave oven using this overheat cutoff switch. This '943 Patent discloses a safety mechanism that protects a microwave oven by detecting an overheating of a cooking chamber and a magnetron via a thermostat. According to this safety mechanism, the thermostat is installed on an appropriate position where it can easily sense the magnetron's ambient temperature and temperatures of the air out of the cooking chamber. If the magnetron's ambient temperature or the temperatures of the air out of the cooking chamber is higher than a predetermined allowable temperature, the power supply from the outside is cut off to provide protection to the microwave oven. FIG. 1 is a block diagram of a conventional microwave oven's abnormal state detecting device employing an overheat cutoff switch.
As shown in FIG. 1, the conventional microwave oven abnormal state detecting device includes a noise filter 2 that removes noise contained in an applied AC power 1; a key input part 5 which receives a key signal input by a user; a first relay RY11 that supplies or cuts off the applied AC power 1; a second relay RY12 that supplies or cuts off the power to a fan motor FM; a magnetron 4 that generates an electromagnetic wave of high frequencies to the cooking chamber; and a high voltage part 3 having a first coil to which the AC power 1 is applied, and a second coil connected to the magnetron 4.
The conventional microwave oven abnormal state detecting device also includes a first therm-switch T1 installed on one side of the bottom of the cooking chamber, and cutting off the applied AC power 1 if the cooking chamber's temperature is higher than a predetermined value; a second therm-switch T2 that is installed near the magnetron 4, and stops the operation of the magnetron 4 if the magnetron 4's temperature is higher than a predetermined value; and a control part 6 that opens or closes the first and second relays RY11 and RY12 to perform the cooking function selected through the key input part 5.
The high voltage part 3 consists of a high-voltage transformer (HVT) that increases the AC power 1 applied from the first coil to a given level at the second coil; a high-voltage fuse (HF) containing a section of conductor which melts when the increased AC power through it exceeds a rated value for a definite period of time; and a high voltage condenser (HVC) and a high voltage diode (HVD) that divide and rectify the AC power increased by the high voltage transformer (HVT) and apply it to the magnetron 4.
The following description concerns the operation of the conventional microwave oven.
If a user opens the microwave oven door and puts a foodstuff in the cooking chamber, and then selects one of various cooking functions by using the key input part 5, the control part 6 turns on the first and second relays RY11 and RY12 to apply the input AC power 1 to the high voltage part 3 and operate the fan motor (FM) in order to cook the foodstuff in the cooking chamber according to the user-input cooking command.
The magnetron 4 emits the electromagnetic wave of high frequencies to the interior of the cooking chamber, thus starting the cooking operation.
If the cooking chamber's temperature becomes higher than a predetermined value, i.e. the cooking chamber is overheated during cooking where the conventional microwave oven's magnetron 4 is operating, the first therm-switch T1 is turned off to shut off the power supply. In addition, if the magnetron 4's temperature becomes higher than a predetermined value during cooking, the second therm-switch T2 is turned off to stop the operation of the magnetron 4. A bimetal that is turned off automatically if the temperature becomes higher a rated value can be used as the therm-switches.
In the conventional microwave oven employing these therm-switches, once a user carelessly inputs a desired cooking command through the key input part in an unloaded state where nothing is in the cooking chamber, the control part does not sense it and operates the magnetron 4 in response to the applied signal. Accordingly, the electromagnetic wave of high frequencies is emitted to the interior of the cooking chamber that does not contain any foodstuff to be cooked, thus damaging the internal components of the microwave oven. In addition, if any one of the overheat cutoff switches, each provided to the cooking chamber and the magnetron, becomes degraded by long-time use, it is not turned off in a good time to cause various dangerous troubles.
In addition, the therm-switch serving as an overheat cutoff switch, must be provided to each of the cooking chamber and the magnetron, thus making the structure of the microwave oven complicated, and increasing the overall production costs.